The Big Blue mouse carries a transgenic integration of 30-40 copies of bacteriophage lambda each carrying a single copy of the E. coli lacI gene, which is used as a reporter for in vivo mutagenesis studies. The goal of these studies is to use the lacI gene to quantitate tissue specific genomic mutations as potential initiating events in carcinogenesis. We have continued our efforts to define the experimental and statistical methods useful for analyzing mutations in vivo. In addition, we are using the Big Blue system to assess the effects of cellular proliferation on the yield of mutations either in the presence or absence of DNA damage. The in vivo mutagenic activity of the carcinogen/noncarcinogen pair, 2,4- or 2,6-diaminotoluene correlates with the carcinogenic potential of these isomers. Cellular proliferation induced by 2,4-DAT appears to be a necessary prerequisite for generating a mutational response in the liver of Big Blue mice. We have also studied the role of proliferation induced by partial hepatectomy on the yield of mutations produced in the liver either with or without pretreatment with the potent mutagen, ENU. ENU is mutagenic in the liver in the absence of hepatectomy; however following hepatectomy, the mutant frequency (MF) was substantially increased in the livers of ENU-treated animals. Notably, hepatectomy alone did not increase the MF in untreated control animals. Under normal circumstances, the target cells appear to be the normally replicating liver cells. Upon hepatectomy, however, most cells in the liver undergo 1-3 cell divisions, thus substantially increasing the number of cells at risk for accumulating a mutation. These data suggest that cell proliferation induced mutations are highly dependent upon the presence of pre-existing DNA damage, thus, implicating pre-existing DNA damage in the etiology of proliferation-induced carcinogenesis.